(1) Field of the Invention
The present invention relates to an optical pickup module, a semiconductor laser device and a semiconductor laser system both incorporated into the optical pickup module, and a method for manufacturing a semiconductor laser system.
(2) Description of Related Art
In recent years, the widespread use of optical disk systems has advanced the increase of the recording density of an optical disk. The optical disk systems have been demanded not only to reproduce data from CDs but also to reproduce data from and record data in write-once CDs (CD-Rs).
By the way, a red laser with a wavelength of a 650 nm band is used to reproduce data from DVDs, and an infrared laser with a wavelength of a 780 nm band is used to reproduce data from and record data in CDs or CD-Rs.
Accordingly, at present, systems for reproducing data from and recording data in DVDs and CDs (RAMBO systems) each require two laser devices of red and infrared semiconductor laser devices and optical components corresponding to the laser devices.
On the other hand, with size reduction of notebook computers or the like, the RAMBO systems have been demanded to become more compact. Therefore, optical pickup modules need become more compact.
To cope with this, a monolithic two-wavelength semiconductor laser system is suggested which is obtained by integrating a red semiconductor laser device and an infrared semiconductor laser device. In the two-wavelength semiconductor laser system, the integration of the two semiconductor laser devices allows the shared use of an optical system and data reproduction and recording with a single optical component. Therefore, optical pickup modules can be made more compact and thinner.
However, crystal growth must be carried out many times to fabricate a monolithic two-wavelength semiconductor laser system obtained by integrating a red semiconductor laser device and an infrared semiconductor laser device on a single substrate. Thus, the number of process steps increases, leading to increased cost.
In order to reduce cost, it is desired that the number of crystal growths is as small as possible. For that purpose, it is preferable that an infrared laser device structure serves as a basis for a semiconductor laser system and a red laser device structure is added to the infrared laser device structure.
However, it is generally difficult to fabricate a red semiconductor laser device made of a material other than AlGaInP-based materials. Hence, it should be considered that a red semiconductor laser device is inevitably made of an AlGaInP-based material.
Thus, an infrared semiconductor laser device need be made of a material that can be grown while being lattice-matched to the AlGaInP-based material.
An infrared semiconductor laser device is typically made of GaAs or AlGaAs that is a material containing As. However, a part of its structure can be made of a material containing Phosphorus (P), such as an AlGaInP-based material, instead of GaAs or AlGaAs, to fabricate an infrared semiconductor laser device. The use of an AlGaInP-based material for an infrared semiconductor laser device permits the simultaneous crystal growths of layers necessary for a red semiconductor laser device and an infrared semiconductor laser device. This does not lead to increase in the number of process steps in fabricating a monolithic two-wavelength semiconductor laser system. Thus, the use of an AlGaInP-based material for an infrared semiconductor is useful for cost reduction. In particular, the use of an infrared semiconductor laser device using a material containing P for cladding layers suppresses the overflow of carriers as compared with the use of cladding layers made of a material containing arsenic (As), thereby obtaining excellent temperature characteristics. Therefore, stable characteristics can be obtained even in a hostile environment such as an environment surrounding a vehicle-mounted infrared semiconductor laser device.
In relation to infrared semiconductor laser devices using a material containing P for cladding layers, there are suggested structures and fabrication methods as disclosed in Japanese Unexamined Patent Publication No. 5-218582 (hereinafter, referred to as Document 1), Japanese Unexamined Patent Publication No. 2001-57462 (hereinafter, referred to as Document 2), and Japanese Unexamined Patent Publication No. 2002-111136 (hereinafter, referred to as Document 3).
Infrared semiconductor laser devices having cladding layers made of a material containing P as disclosed in Documents 1 and 2 are each fabricated on a single-crystal substrate by successively growing the crystals of a cladding layer of a first conductive type, an active layer and a cladding layer of a second conductive type.
However, in the infrared semiconductor laser devices, a GaAs-based or AlGaAs-based material need be used for an active layer because of a desired emission wavelength. In this case, it is difficult to obtain excellent crystallinity on the interface between a cladding layer made of a material containing P and an active layer made of a material containing As.
FIG. 13 illustrates an energy band diagram of a known infrared semiconductor laser device having cladding layers made of a material containing P. As seen from FIG. 13, the energy difference between each cladding layer and the optical guide layer contacting the cladding layer is secured, because the optical guide layer is also composed of a material containing phosphorus (P), specifically, GaInP.
By the way, in an actual fabrication process, a cladding layer, an optical guide layer and an active layer are typically successively grown by metal organic chemical vapor deposition (hereinafter, referred to as MOCVD) or the like. When the cladding layers and the optical guide layers are formed of a material containing P, the source gas need be switched from a gas containing P to a gas containing As in growing a well layer constituting the active layer.
In this case, the gas containing P and the gas containing As coexist in a reactor. This causes a loss of the steepness of composition change or the like at the interfaces between the optical guide layers and the active layers. Therefore, the following problems occur: an injection current becomes uneven; and a multi-quantum well layer has a longer wavelength than the designed wavelength and thus emission intensity becomes extremely feeble.
The active layer determines the characteristics of the semiconductor laser element, such as wavelength and lifetime, and is therefore required to have excellent crystallinity.
However, it turned out that when a cladding layer made of (Al0.5Ga0.5)0.5In0.5P is grown on the substrate and then a multi-quantum well layer made of GaAs/(Al0.5Ga0.5)0.5In0.5P is grown on the cladding layer, the substrate surface is whitened and therefore the designed wavelength cannot be obtained.
It is considered that the reason for this is that since the multi-quantum well layer was grown with a gas containing P and a gas containing As mixed, its epitaxial growth has been done unsuccessfully. When the crystallinity of the active layer is thus impaired due to a cross contamination in switching the source gas, the device reliability decreases in lifetime and resistance regardless of the obtainment of the designed wavelength.
Document 3 discloses an example in which an AlGaAs-based optical guide layer is placed between an AlGaInP-based cladding layer and an active layer. In this case, a multi-quantum well layer has a layered structure composed of a well layer made of AlxGa1−xAs (x≦0.15) and a barrier layer made of In0.5(Ga1−xAlz)0.5P (0≦z≦0.2). Therefore, the source gas need be switched from a gas containing P to a gas containing As. Hence, the problem that excellent crystals cannot be obtained is not solved.